In a power semiconductor element for high-power control used to control wind power generation, a transportation system such as an electric automobile and the like, a large amount of heat is generated, and thus, as a substrate mounting the power semiconductor element thereon, for example, a power module substrate in which metal sheets having excellent conductive properties are bonded to both surfaces of a ceramic substrate made of aluminum nitride (AIN) as a circuit layer and a metal layer is widely employed. Furthermore, in the above-described power module substrate, in some cases, a heat sink is bonded to the metal layer side through a solder material.
For example, a power module substrate described in Patent Document 1 has a structure in which copper sheets are bonded to both surfaces of a ceramic substrate. In such a power module substrate, the copper sheets are bonded to both surfaces of the ceramic substrate by disposing the copper sheets on the surfaces through an Ag—Cu—Ti-based brazing filler metal and carrying out a heating treatment.
In a case in which the metal layer and the heat sink in the power module substrate disclosed in Patent Document 1 are bonded together through the solder material, the loading of a thermal cycle causes the generation of thermal stress due to the difference in thermal expansion coefficient between the power module substrate and the heat sink. However, since the metal layer is made of copper having strong deformation resistance, it is not possible to absorb the thermal stress through the deformation of the metal layer, and thus there is a concern that cracks may be generated in the ceramic substrate.
Therefore, Patent Document 2 proposes a power module substrate in which a copper sheet is bonded to one surface of a ceramic substrate so as to form a circuit layer and an aluminum sheet is bonded to the other surface so as to form a metal layer. In a case in which the metal layer of this power module substrate and a heat sink are bonded together, thermal stress generated between the power module substrate and the heat sink due to the loading of a thermal cycle is absorbed by the metal layer made of aluminum having relatively small deformation resistance, and thus it is possible to suppress the generation of cracks in the ceramic substrate.
In the power module substrate described in Patent Document 2, a heating treatment is carried out with a Ag—Cu—Ti-based brazing filler metal interposed on one surface of the ceramic substrate so as to bond the copper sheet, and then another heating treatment is carried out with an Al—Si-based brazing filler metal interposed on the other surface of the ceramic substrate so as to bond the aluminum sheet.